We have found that half of the babies gestated at 3100 m (Leadville, Colorado) fail to hyperventilate when exposed to hypoxia, but response is similar to sea level babies when in a hyperoxic environment. This failure to react to hypoxia disappears by 15 days of age and the reaction resembles the adult pattern. These results explain the high morbidity seen in the Leadville nursery. Use of 30 percent oxygen during the stay in the nursery has cut down morbidity and transfer to Denver. Serial measurements of effective pulmonary blood flow have been found to be high at 12 hours of age but becomes lower than seen in Denver and sea level infants. No explanation has been found but other indirect measurements of cardiac output (echocardiography) are being planned. Lung growth measurements have been found to be the same as sea level babies. The explanation for the differences seen in these altitude babies have not been explained, but preliminary evidence seems to indicate that some mothers who are gestating their pregnancy at 3100 meters are slightly hypoxic, and their babies lose the sea level type response to hypoxia.